In recent years, various sizes of optical elements have been manufactured and put into market, for example, from large size optical elements utilized as such as a telescope lens and a microscope lens to small size optical elements, for example, pickup lenses of such as CD (Compact Disc) and DVD (Digital Video Disc). Specifically, development of an optical pickup lens (an objective lens) applicable to at least one of BD (Blue-ray Disc), CD and DVD has been actively carried out in accordance with a proposal of a standard on BD or AOD (Advanced Optical Disc). On these optical elements, a functional layer depending on applications is generally formed to obtain a specific property determined by a combination of the function of the layer and a wavelength range of light irradiated to the functional layer.
For example, in Patent Document 1, disclosed is an optical element on which an antireflection layer is formed, and the antireflection layer works on a transmitting light of an ultraviolet region. On this optical element, a film of a high refractive index material (12) prepared from neodymium fluoride is formed directly on a synthetic quartz substrate (11) and a film of a low refractive index material (13) prepared from aluminum fluoride is formed directly on the high refractive index film. In Patent Document 1, it has been proved that the optical element exhibited a sufficient antireflective function (refer to paragraph Nos. 0016–0019, FIGS. 1 and 2).
Since ultraviolet light has a short wavelength and large energy of light, a technique to provide a film exhibiting a different refractive index on the surface of a lens body, as disclosed in Patent Document 1, is useful to obtain an antireflective function. However, an objective lens having a high refractive index antireflection film on a lens body made of a resin tends to have a problem in that the lens surface facing to an optical information recording medium is deformed or abrasion resistance of the antireflection film decreases due to a local heat generated at the surface and subsequently conveyed to the lens body, the local heat being generated when the antireflection film absorbs the energy of light at the surface facing to the optical information recording medium side surface. This may be a bigger problem for an antireflection film having an refractive index of 1.7 or more. When the generated local heat is conveyed to the lens body, due to the difference in thermal expansion coefficients of the antireflection film and the lens body, the abrasion resistance of the antireflection film may be reduced. Further, when an additive is used in a resin forming a lens body, oxidation or deterioration of the additive may occur and enhances the reduction of abrasion resistance or deformation of the lens surface.
(Patent Document 1) Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 8-179103.